Apparatus and method for separating gases

ABSTRACT

An apparatus and method to separate a mixture of gases—such as carbon dioxide and methane—by means of an inorganic membrane comprising a ceramic support and a silica layer. The invention can efficiently separate the gaseous mixture and can also cope with the extreme conditions found in e.g. hydrocarbon producing wells. A method of manufacturing the apparatus is also disclosed.

[0001] This invention relates to an apparatus and method for separatinggases and in particular an apparatus comprising inorganic membranes forremoving acidic gases from natural gas.

[0002] Natural gas reserves known to contain a relatively high contentof nitrogen, carbon dioxide or hydrogen sulphide are rarely recovereddue to the costs incurred to purify the gas mixture.

[0003] Impure methane is also commonly produced by landfill sites butits commercial exploitation is generally is prohibited by the costsassociated with purifying it.

[0004] Current processing systems are generally regarded to beuneconomical above 1.5% levels of carbon dioxide. To remove carbondioxide from natural gas, chemical scrubs are commonly used. Thisresults in a significant amount of waste product which must be suitablydisposed of, adding further costs to remove the carbon dioxide.

[0005] Moreover, the mechanical equipment used with such chemical scrubsis susceptible to failure.

[0006] According to a first aspect of the present invention, there isprovided an apparatus to separate at least one first gas from a mixturecomprising the at least one first gas and at least one second gas, theapparatus comprising a membrane adapted to permit passage of the atleast one first gas therethrough whilst substantially preventing passageof the at least one second gas therethrough.

[0007] Preferably, the membrane is an inorganic membrane.

[0008] The first gas may be water vapour, nitrogen or preferably carbondioxide.

[0009] Preferably, the apparatus separates a gas mixture comprisingnatural gas an acidic gas, and typically, the acidic gas is the firstgas and the natural gas is the second gas.

[0010] The acidic gas in preferred embodiments is carbon dioxidealthough other acidic gases such as hydrogen sulphide, may be the firstgas.

[0011] Preferably, the inorganic membrane is formed such that itmaximises the contact of the gaseous mixture with the surface of theinorganic membrane. Preferably, the inorganic membrane is provided as atube comprising a bore. Optionally, a series of tubes may be used, andthe tube may be corrugated or coiled. The gaseous mixture may bedirected through the bore of the tubes and separated according to thepresent invention.

[0012] More preferably, each tube is an inner tube provided within animpermeable second outer tube. Preferably, the gaseous mixture isinjected into the annulus between each pair of outer and inner tubes.

[0013] Preferably, a graphite seal mounts each inner tube in each outertube.

[0014] Preferably, the inorganic membrane comprises a means to controlthe type of gaseous molecules passing therethrough. Preferably the outerdiameter of the inner tubes is between 5-12 mm, more preferably 10 mm.Preferably the thickness of the inner tubes is between 1.5 and 2 mm,more preferably 1.7 mm. There may be any number of inner tubes althoughpreferably there are between 10 and 50 inner tubes depending on the flowrate and the purity of the gaseous mixture. Preferably, the inner tubesare approximately 1 metre in length.

[0015] Preferably, the inorganic membrane comprises a plurality ofchemically discreet portions. Preferably, a first portion is aseparating layer. Preferably, a second portion is a support.

[0016] Preferably, the separating layer comprises a layer adapted toallow passage of the at least one first gas through the membrane andresist passage of the at least one second gas through the membrane.

[0017] Preferably, the separating layer comprises any one of, acombination of, or all of; silica, magnesium oxide, gamma alumina or amolecular sieve. Preferably, the molecular sieve is a carbon molecularsieve.

[0018] The support may comprise alpha alumina, stainless steel, carbonor any other suitable inorganic material.

[0019] Preferably, the separating layer is provided on a surface of thesupport, and where the support is a tube, the separating layer may beprovided on a surface of the inner bore of the tube.

[0020] The layer(s) of the separating layer may be provided in any orderalthough in preferred embodiments, a layer of gamma alumina is firstadded to the support. Typically, a layer of silica is then added on topof the layer of gamma-alumina. Optionally, a molecular sieve may beadded as a further layer.

[0021] More preferably, the separating layer has a chemical affinity forthe at least one first gas. A group II metal oxide, preferably magnesiumoxide, may be added, optionally in place of the silica and molecularsieve, to increase the chemical affinity of the at least one first gastowards the membrane.

[0022] According to a second aspect of the present invention, there isprovided an apparatus to separate at least one first gas from a mixturecomprising the at least one first gas and at least one second gas, theapparatus comprising a first tube and a second tube, the first tubecomprising a membrane adapted to permit passage of the at least onefirst gas therethrough whilst substantially preventing passage of the atleast one second gas therethrough, the first tube being mountedsubstantially within the second tube and being sealed therein by agraphite seal.

[0023] Preferably, the membrane is the membrane according to the firstaspect of the invention.

[0024] According to a third aspect to the present invention, there isprovided a method of manufacturing apparatus to separate at least onefirst gas from a mixture comprising the at least one first gas and atleast one second gas, the apparatus comprising an membrane adapted topermit passage of the at least one first gas therethrough whilstsubstantially preventing passage of the at least one second gastherethrough, the method comprising

[0025] providing a support;

[0026] immersing the support in a sol;

[0027] removing the support from the sol; and

[0028] allowing the support to dry.

[0029] Preferably, the membrane is an inorganic membrane.

[0030] Preferably, the support is a ceramic support.

[0031] Preferably, the membrane manufactured according to the secondaspect of the invention is the membrane provided according to the firstaspect of the invention.

[0032] Preferably, the following steps of the method

[0033] immersing the support in a sol;

[0034] removing the support from the sol; and

[0035] allowing the support to dry;

[0036] are repeated at least once. More preferably, said steps of themethod are repeated twice.

[0037] Preferably, the sol is in the liquid state and forms at least aportion of the separating layer. Preferably, the sol coats the support.Preferably, the sol forms at least a part of the separating layer.

[0038] Preferably, the support is dried by applying heat.

[0039] In certain embodiments, the method may be repeated to coat thesupport with a second sol.

[0040] Optionally, the support may be coated with a molecular sieve,preferably a carbon molecular sieve, instead of, although preferably, inaddition to, other sols. In such embodiments carbonisation is preferablyaffected by heating the support with the carbon molecular sieve in anargon atmosphere.

[0041] According to a fourth aspect of the present invention, there isprovided a method to separate at least one first gas from a mixturecomprising the at least one first gas and at least one second gas, themethod comprising the steps of

[0042] bringing the said mixture into contact with a membrane;

[0043] allowing passage of the at least one first gas through themembrane whilst substantially preventing passage of the at least onesecond gas through the membrane.

[0044] Preferably, the membrane is an inorganic membrane.

[0045] Preferably, the method according to the third aspect of theinvention is used in conjunction with the apparatus according to thefirst aspect of the invention.

[0046] In certain embodiments of the invention, the method is performedin a downhole environment.

[0047] Typically, the at least one first gas includes an acidic gas.Preferably, the at least one first gas includes carbon dioxide. Morepreferably, the at least one first gas and the at least one second gasmay be recovered, suitable for use with alternative applications.

[0048] Typically, the at least one second gas includes a hydrocarbongas. Preferably, the at least one second gas includes methane.Preferably, the said mixture is essentially a mixture of methane andcarbon dioxide.

[0049] Alternatively, the apparatus and method may be used to removecarbon dioxide from nitrogen. The apparatus and method according to anyaspect of the invention may also be used to separate other gas, fluid,or liquid mixtures, for example, to remove hydrogen sulphide frommethane.

[0050] Embodiments of the present invention will now be described by wayof example only, with reference to the following diagram, wherein:

[0051]FIG. 1a is a side view of an inorganic membrane according to thepresent invention;

[0052]FIG. 1b is an enlarged side view of the inorganic membraneaccording to the present invention; and,

[0053]FIG. 2 is a diagrammatic view of a tube comprising the inorganicmembrane;

[0054]FIG. 3 is a graph showing the recovery and separation factor foran inorganic membrane in accordance with the present invention fordifferent concentrations of carbon dioxide in a feed gas;

[0055]FIG. 4 is a graph showing the effect of deposition time onthickness of a silica membrane in accordance with the present invention;

[0056]FIG. 5 is a schematic view of an inorganic membrane in accordancewith the present invention showing the permeation or retention ofvarious molecules;

[0057]FIG. 6a is a first electron micrograph output showing thestructure of an inorganic membrane in accordance with the presentinvention at a magnification of 2,500;

[0058]FIG. 6b is a second electron micrograph output of the inorganicmembrane at a magnification of 1,000.

[0059]FIG. 7 is a side view of a tube comprising the inorganic membrane.

EXAMPLE 1

[0060]FIGS. 1a and 1 b show an inorganic membrane 1 in accordance withthe present invention. In summary, the membrane 1 is used to removecarbon dioxide CO₂ from a gaseous mixture comprising methane CH₄ andcarbon dioxide CO₂ in accordance with the present invention. Theinorganic membrane 1 comprises a relatively highly porous ceramicsupport 2 and a separation layer 3.

[0061] The support 2 is a coarse porous support, and this firstpreferred example of support 2 comprises 76% alpha-alumina and 23%titania, the support 2 typically having a pore size of 500 nm and aporosity of 45%. Such a support 2 is commercially available, buthitherto has only been used as a filter for microfiltration. The support2 may alternatively be made from any other suitable material, forexample, silicon carbide, zirconia, stainless steel or carbon.

[0062] The separating layer 3 of Example 1 is a layer of silica 3.

[0063] The membrane 1 is prepared by a repeated dip-coating technique.The support 2 is repeatedly dipped into a precursor or “sol” (not shown)and dried to form an evaporated layer of sol on the support 2, therebyforming the membrane 1.

[0064] The sol is prepared by mixing nine parts of isopentane and onepart silicon elastomer, to obtain a clear and colourless sol. A curingagent such as one from the Sylgard® series is then added equivalent toone-tenth of the elastomer and the resulting sol mixed at roomtemperature.

[0065] The sol is permitted to age over a period of 5-30 minutes (mostpreferably 20 minutes), and thereafter, the support 2 is immersed intothe aged sol for approximately 20 minutes. The sol is then drained andevaporated from the support 2 by drying the support 2 at 65° C. for 24hours in an oven to form a layer on the support 2. The procedure isrepeated a number of times until the layer is of the required thickness,normally between 1-12 μm, preferably 6 μm. FIG. 4 shows the membranethickness in relation to deposition time and No. of dips.

[0066] When a CO₂ molecule collides with the separation layer 3 of theinorganic membrane 1, it may be adsorbed into the separation layer 3 andproceed through the pores 5 in the support 2. The CO₂ molecule continuesthrough the support 2 and is recovered along with other CO₂ molecules byany suitable means.

[0067] In contrast, when a CH₄ molecule collides with the separationlayer 3 of the inorganic membrane 1, it is unlikely to be adsorbed intothe separation layer 3 and will instead continue through the bore of thetube of the inorganic membrane 1 where it may be collected along withother CH₄ molecules. Generally, CH₄ molecules form bonds with theseparation layer 3 less readily than CO₂ molecules.

[0068] Carbon dioxide is currently injected downhole to increase therate of recovery of production fluids from reservoirs. The presentinvention therefore provides a means to obtain carbon dioxide proximateto where it may be used. A continuous loop is thus formed in whichcarbon dioxide is recovered from the natural gas and may be utilised torecover further production fluids. Indeed the CO₂ may never need to betransported to the surface as it may be transported from the membrane 1to the reinjection point which saves on further time and cost.

[0069] The inorganic membrane may be formed as a flat sheet orpreferably, in thin tubes having an inner diameter of, for example 3-11mm and an outer diameter of for example 5-12 mm. The gaseous mixture 4is directed through the inner bore of this tube membrane 1. Such tubesmay be coiled or corrugated to increase the number of collisions betweenthe molecules in the gaseous mixture 4 and the inner surface of theinorganic membrane 1.

[0070] An example of the tube arrangement suitable for use in accordancewith the present invention is shown in FIG. 7. A tubular stainless steelvessel 9 comprises an inner tube 11, inlet ports 13, 17 and a seal 12.The inner tube 11 is made from the inorganic membrane 1 and the outertube 10 can be made from any suitable material, such as stainless steel.Normally approximately 10 tubes are used in any one vessel 9, althoughonly one end 15 of one tube is shown in FIG. 7. Certain embodiments ofthe invention with high flow rates may use more than 10 tubes. The seal12 is preferably made from graphite as this is compressible, inert,high-temperature resistant to enable permeability studies at elevatedtemperatures, and cost effective. The second end 16 of the vessel 9 isnot shown in FIG. 7, but typically mirrors the configuration of thefirst end 15.

[0071] The gaseous mixture 4 is injected through the inlet port 13 intothe annulus 14 between the inner 11 and outer 10 tubes. In this exampleCO₂ molecules are separated from CH₄ molecules, but other mixtures maybe separated. The mixture 4 flows through the annulus 14, the CO₂molecules selectively adsorbing in the inorganic membrane 1 which formsthe tube 11. The second end 16 of the inner tube 11 of the vessel 9corresponds to a first outlet (not shown), and the second end 16 of theannulus 14 of the vessel 9 corresponds to a second outlet (not shown).The relatively pure CH₄ and CO₂ are recovered separately through theirrespective outlets. A sweep gas 18 may optionally be injected throughthe inlet 17 of the inner tube 11, to increase the flow rate of the CO₂therein.

[0072] A simplified embodiment of the tube 11 is shown in FIG. 2 withlike parts labelled correspondingly.

[0073] The efficiency of the membrane 1 in separating CO₂ from naturalgas is dependent on both the geometry in terms of surface area and flowrate and the membrane 1 characteristics. The tests conducted to datehave only considered the factors relating to the membrane and notoptimised the geometry. Hence a standard test set up can be used for alltesting.

[0074] Thus the measures of selectivity and efficiency relate to astaged separation factor. The tests are conducted in two stages usingthe equipment as per FIG. 2 or FIG. 7.

[0075] In a first test, a feed gas was introduced in known CO₂ and CH₄composition and mass flow rates and the permeate gas is fed through aflow meter and the concentration of the permeate is analysed ignoringthe sweep gas. This gives a measure of the staged separation factor. Theretentate gas is free to exhaust.

[0076] To calculate the Staged Separation Factor (SSF), the followingformula is used:

SSF=(Conc. Of CH₄/Conc. Of CO₂ in permeate)/(Conc. Of CH₄/Conc of CO₂ infeed)×100

[0077] Therefore the lower the SSF the better is the separationefficiency of the separating means (in this case membranes).

[0078] In the second test, a feed gas is introduced in known CO₂ and CH₄composition and mass flow rates and the retentate gas is fed through aflow meter and the concentration of the retentate is analysed ignoringthe sweep gas. This gives a measure of the Staged Recovery Factor. Thepermeate gas is free to exhaust.

[0079] The SRF is calculated using the following formula

SRF=(Conc. Of CH₄/Conc. Of CO₂ in retentate)/(Conc. Of CH₄/Conc of CO₂in feed)×100

[0080] Therefore the higher the SRF, the better is the efficiency of theseparating means (in this case membranes.) Parameters affectingseparation efficiency are discussed in Industrial Gas Separations, pp132-134 (Schell & Houston) and Gas Purification, Membrane PermeationProcesses pp 1242-1245.

[0081] SSF and SRF measurements on the present example of an inorganicmembrane is shown in FIG. 3.

[0082] The measurements were taken under conditions of 1 atmosphere andfor a relatively small tube. It is expected that the selectivity ofseparating CO₂ from CH₄ will increase when the pressure is increased.Moreover, use of longer tubes or two—three smaller tubes in series willalso increase selectivity.

EXAMPLE 2

[0083] An inorganic membrane 1 according to the invention comprises aporous ceramic support 2 and a separation layer 3, as shown in FIGS. 1aand 1 b.

[0084] Example 2 differs from Example 1 only in the composition ofseparation layer 3 provided. Common features between Example 1 andExample 2 are not described here for example 2.

[0085] The separation layer 3 of the Example 2 comprises a gamma-aluminalayer (not shown) mounted on the support 2, a silica layer (not shown)and a carbon molecular sieve (not shown).

[0086] To form the membrane 1 of the Example 2, the support 2 is exposedto a boemite sol maintained at 0.6 mol/L (as the gamma alumina source)using the dip-coating technique as described for example 1. The support2 is immersed into the boemite sol for approximately two minutes. Themembrane is then air dried overnight and heated to between 700 and 800°C. at a rate of 1° C./minute. The process is normally repeated threetimes or more to achieve the required thickness of gamma alumina on thesupport 2, normally between 1-12 μm, preferably 6 μm.

[0087] Once the required gamma-alumina layer thickness has been added tothe support 2, a silica or carbon monocular sieve layer is then appliedto form the final separating layer 3. The silica-layer is deposited overthe gamma alumina by the method described above in relation to Example1.

[0088] To deposit the carbon monocular sieve layer, the alumina/silicacoated support 2 is dipped in a polyetherimide solution of between 1 and5 moll⁻¹, preferably 3 moll⁻¹. The support 2 is then dried in air.Carbonisation is performed in an argon atmosphere using a predefinedtemperature profile. In this example, the support was heated from 20-80°C. for 2 hours and then from 80-120° C. for 4 hours although a varietyof temperature profiles may be suitably employed. Such a process may berepeated as necessary to achieve the required CO₂/CH₄ selectivity andCO₂ permeability.

EXAMPLE 3

[0089] An inorganic membrane 1 according to the invention comprises aporous ceramic support 2 and a separation layer 3, as shown in FIGS. 1aand 1 b.

[0090] Example 3 differs from Example 1 only in the composition ofseparation layer 3 provided. Common features between Example 1 andExample 3 are not described here for Example 3.

[0091] The separation layer of Example 3 does not comprise a silicalayer in contrast to the previous Examples 1 and 2. A layer ofgamma-alumina is added directly onto the support 2 as detailed forExample 2.

[0092] The support 2 is then chemically modified by impregnating itssurface using magnesium nitrate, Mg(NO₃)₂.Mg(NO₃)₂ reduces to form MgOwhich is thus located in the pores of the separating layer so that thesurface concentration is 4 mmols Mg per square metre.

[0093] The chemical affinity between the magnesium oxide and the carbondioxide increases the selectivity of the membrane 1.

[0094] The ceramic nature of the inorganic membrane 1 may be used athigh temperatures and pressures and in extreme conditions, for exampledownhole. Moreover, the ceramic materials are resistant to acidicdegradation; acids such as carbonic acids being commonly formedthereabouts by the combination of CO₂ and H₂O. Therefore, embodiments ofthe invention can be used to separate mixtures of ‘wet’ gases whichwould degrade other separating means. Ceramic materials also have a highmechanical strength.

[0095] Embodiments of the invention used downhole have the advantagethat acidic gases are removed before transfer by pipeline therebyreducing the corrosion of the pipeline caused by such acidic gases.

[0096] The passage of CO₂ through the membrane 1 enables continuousproduction of a relatively pure methane at high pressure making theprocess extremely cost effective.

[0097] Most territories impose restrictions on releasing theenvironmentally damaging CO₂ to the atmosphere and so certainembodiments of the present invention provide a means to remove this gasfrom natural gas before flaring.

[0098] The apparatus according to the present invention may also be usedin exhaust stacks to remove, for example, CO₂ from exhaust fumes. Forexample, the impure methane produced from landfill sites may be purifiedon site and then used as a fuel.

[0099] The hybrid structures may be characterised by X-ray diffraction,scanning electron microscopy(SEM), nitrogen absorption, X-rayphotoelectron spectroscopy, BET surface analysis and EDAX surfaceelemental analysis. SEM photographs are shown in FIGS. 6a and 6 b at2500 and 1000 times magnification respectively.

[0100] Embodiments of the invention may be used to separate othergaseous or fluid mixtures, e.g. N₂ or H₂S may be separated from rawnatural gas at mildly high temperatures of ˜50-100° C. This is permittedby the relative molecular dimensions of CH₄, N₂, CO₂, H₂O and H₂S whichare summarised in the table below Molecule Size Å CH₄ 3.8 CO₂ 3.3 N₂ 3.6H₂0 2.7 H₂S 3.6 (Membrane) 3.6-3.7 A

[0101] Thus, even though CH₄ is lighter than the other molecules and sowould be expected to penetrate the membrane more readily than theheavier molecules, it has been found that membranes according to theinvention allow passage of the heavier molecules while restrictingpassage of the lighter methane molecules.

[0102] Preferably, the selectivity of CO₂/CH₄ is 150 at 350° C.; that is150 CO₂ molecules will travel through the membrane for each CH₄ moleculethat travels through the membrane. Preferably, the selectivity of CO₂/N₂is 120 at 350° C. Preferably, the permeability of CO₂ through themembrane 1 is >4×10⁻⁷ mol/m² sPa at 350° C. Preferably, the durabilityof the inorganic membrane 1 is greater than 500 hours at 350° C. incorrosive environments.

[0103] An advantage of using ceramic membranes to purify natural gas istheir durability. Absorbent performance of known separating meansgenerally decrease with their age whereas the absorbent performance ofceramic materials do not decrease with age. Embodiments including asilica layer are particularly durable. Further advantages of the use ofceramics in such applications may include enhanced plant performance anda reduction in energy consumption. Ceramic materials may also be usedfor mixtures with high CO₂ concentrations for example, ranging from 3%to 72% CO₂.

[0104] Improvements and modifications may be made without departing fromthe scope of the invention.

1. An apparatus to separate at least one first gas from a mixturecomprising the at least one first gas and at least one second gas, theapparatus comprising a membrane adapted to permit passage of the atleast one first gas therethrough whilst substantially preventing passageof the at least one second gas therethrough.
 2. An apparatus as claimedin claim 1, wherein the membrane is a ceramic membrane.
 3. An apparatusas claimed in any preceding claim, wherein the first gas comprises anacidic gas and the second gas comprises methane.
 4. An apparatus asclaimed in any preceding claim, wherein the first gas comprises carbondioxide.
 5. An apparatus as claimed in any preceding claim, wherein themembrane comprises at least one tube having a bore.
 6. An apparatus asclaimed in claim 5, wherein the at least one tube is corrugated orcoiled.
 7. An apparatus as claimed in claim 5 or 6, wherein the at leastone tube comprises an inner tube provided within an impermeable secondouter tube and the mixture comprising the at least one first gas and atleast one second gas is injected into an annulus between the inner andouter tubes.
 8. Apparatus as claimed in claim 7, wherein a graphite sealmounts the inner tube in the outer tube.
 9. Apparatus as claimed in anypreceding claim, wherein the membrane comprises at least one of silica,magnesium oxide, gamma alumina and a molecular sieve.
 10. Apparatus asclaimed in claim 9, wherein the molecular sieve is a carbon molecularsieve.
 11. Apparatus as claimed in any preceding claim, wherein themembrane comprises a separating portion adapted to allow passage of theat least one first gas through the membrane and substantially resistpassage of the at least one second gas through the membrane and, asupport portion.
 12. Apparatus as claimed in claim 11, wherein thesupport portion comprises at least one of alpha alumina, stainless steeland carbon.
 13. Apparatus as claimed in claim 11 or claim 12, whereinthe separating portion is provided on a surface of the support. 14.Apparatus as claimed in claim 13, wherein the separating portioncomprises a layer of gamma alumina and a layer of silica.
 15. Apparatusas claimed in claim 14 when dependent upon any of claims 11 to 13,wherein the layer of gamma alumina is provided on the support portionand the layer of silica is provided on the layer of gamma alumina. 16.Apparatus as claimed in any of claims 11 to 15, wherein the separatingportion has a chemical affinity for the at least one first gas. 17.Apparatus as claimed in any preceding claim, wherein a group II metaloxide is added to the membrane to increase the chemical affinity of theat least one first gas toward the membrane.
 18. Apparatus as claimed inclaim 17, wherein the group II metal oxide is magnesium oxide.
 19. Amethod of manufacturing apparatus as claimed in any preceding claim, themethod comprising providing a support; immersing the support in a sol;removing the support from the sol; and allowing the support to dry. 20.A method as claimed in claim 19, wherein the following steps of themethod immersing the support in a sol; removing the support from thesol; and allowing the support to dry; are repeated at least once.
 21. Amethod as claimed in any one of claims 19 or 20 when dependent upon anyof claims 11 to 16, wherein the sol forms at least part of theseparating portion.
 22. A method as claimed in any one of claims 19 to21, wherein the support is dried by applying heat.
 23. A method asclaimed in any one of claims 19 to 22 which is repeated to coat thesupport with a second sol.
 24. A method as claimed in any of claims 19to 23 when dependent on claim 10, wherein carbonisation is affected byheating the support with the carbon molecular sieve in an argonatmosphere.
 25. A method to separate at least one first gas from amixture comprising the at least one first gas and at least one secondgas, the method comprising the steps of bringing the said mixture intocontact with a membrane; such that the at least one first gas passesthrough the membrane whilst passage of the at least one second gasthrough the membrane is substantially prevented.
 26. A method as claimedin claim 25, wherein the membrane is an inorganic membrane.
 27. A methodas claimed in claim 25 or claim 26, which is performed in a downholeenvironment.
 28. A method as claimed in any one of claims 25 to 27,wherein the at least one first gas and the at least one second gas arerecovered for subsequent use.
 29. An apparatus to separate at least onefirst gas from a mixture comprising the at least one first gas and atleast one second gas, the apparatus comprising a first tube and a secondtube, the first tube comprising a membrane adapted to permit passage ofthe at least one first gas therethrough whilst substantially preventingpassage of the at least one second gas therethrough, the first tubebeing mounted substantially within the second tube and being sealedtherein by a graphite seal.